Increasingly, designers of devices that use laser light, such as laser printers and optical memories, have found it advantageous to use arrays of closely spaced laser diodes as their light source. Closely spaced diodes allow for multiple beam processing and thus improve data throughput as compared with older systems that employ continuous wave, single beam gas lasers.
Currently, two array architectures have emerged for use in various applications--monolithic and nonmonolithic laser diode arrays. Typically, monolithic diode arrays are manufactured as an integral array unit. As a result, the diodes comprising the unit usually have the same light characteristics, such as wavelength and polarization.
By contrast, nonmonolithic arrays usually comprise a plurality of individual laser diodes mounted on a support. Individually manufactured diodes offer the advantage of variable beam characteristics. For example, each diode in the array may lase at a different wavelength or polarization. This variation is desirable in the context of color laser printing, as discussed in U.S. Pat. No. 5,243,359 issued to Tibor Fisli on Sep. 7, 1992, entitled "A Raster Output Scanner for a Multistation Xerographic Printing System."
Typically, the lasing elements in both monolithic and nonmonolithic laser arrays are individually addressable. Individual addressability generally requires that each lasing element have a separate current source that drives or modulates the lasing element. In operation, each driver sends a current through the diode sufficient to induce emission of laser light. The amount of current the driver produces is determined, in part, by the digital data driving that particular lasing element.
However, because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the amount of output power from each laser diode. If it is found that a particular diode is outputting too much or too little power at a given current level then the current needs to be adjusted to correct for the power differential.
Diodes are typically constructed layer by layer from epitaxial deposition of appropriately doped semiconductor material. The front and back facets are then cleaved to produce reflective surfaces that define the front and back boundaries of the laser cavity. The front facet is designed to be more transmissive than the back facet which is designed to be more reflective. In that way, the front facet is the side from which the majority of laser light is emitted.
As stated above, the back facet is designed to be a highly reflective surface. However, some light ultimately escapes through the back facet of the diode. The amount of light leakage through the back facet is generally known to be proportional to the amount of light emitted from the front facet. This relationship between radiation from the back facet and the radiation from the front facet affords the opportunity to monitor the amount of output power from the front facet.
To measure the amount of light from the back facet of an array of diodes, a detector is typically disposed opposite the back facet of a single laser diode. In the case of a single laser diode configuration, one back facet detector gives complete information concerning the amount of radiation emanating from the front facet of that diode. In a multi-diode configuration, the confluence of concurrent, multiple beams does not give information concerning any particular diode.
FIG. 1 depicts a known laser diode array design 10. Laser diodes 20 are disposed on support 25 so as to maintain close spacing of their output beams 30 from their respective front faces. While the majority of the laser light escapes from the front facet, some radiation 35 leaks from the back facet of the diode.
As can be seen from FIG. 1, the light from both the front and the back facet spread out in a conic shape and overlap at some distance from the facets. Thus, the radiated beams emanating from the back facets of the closely spaced diodes are difficult to distinguish. This problem becomes more acute as the diodes are spaced closer together.
Separating and detecting light from individual diodes is important from the standpoint of monitoring and ultimately controlling the output power from individual diodes. With arrays of closely spaced laser diodes, this is generally not possible with a single back facet detector.
The problem with a single back facet detector opposite the entire set of diodes is that no discernible information concerning the output power of any one laser diode is available when two or more beams are concurrently impinging upon the detector. Without information concerning individual diodes, it is not possible to correct for individual variations in output.
Thus, there is a need to construct an array architecture such that the amount of light emitted from individual back facets is detected. Additionally, there is a need to regulate the output of the individual diodes in a continuous closed loop fashion, given this information concerning output power. This regulation is needed to insure high print quality.
It is thus an object of the present invention to provide an array architecture such that the amount of output power from individual back facets of laser diodes can be individually monitored in a continuous fashion.